Composition and method for coating stainless steel articles



United States Patent 3,130,087 COMPOSITION AND METHOD FOR COATINGSTAINLESS STEEL ARTICLES Ira 3'. Duncan, Atlanta, Ga., assignmto DetrexChemical Industries, Inc, Detroit, Mich, a corporation of Michigan NoDrawing. Filed May 1, 1961, Ser. No. 155,223 (Filed under Rule 47(1))and 35 U.S.C. 118) 8 Claims. (Cl. 1486.24)

This invention relates to an improved composition and method for formingtightly adherent coatings on metal articles which are to be subjected toplastic deformation. More particularly, it relates to the formation, onferrousbased stainless metal articles of tightly adherent oxalatecoatings on the surfaces thereof.

It is known to coat ferrous-based stainless metals such as stainlesssteel and the like with an adherent surface coating of an oxalatecompound prior to plastically deforming such articles by such methods asdrawing and stamping. However, all of the effective known prior arttreatments require either the utilization of a complex pre-treatment ofthe stainless surfaces, or the use in the treating bath of expensiveand/ or dangerous compounds. For example, one prior art process involvesa pro-treatment comprising an acid pickle and alkaline rinse. Thisincidentally requires the use of additional water rinses, and in theevent that the alkaline rinse contains the conventional alkalimetalcyanide, a further water rinse is required to obviate theliberation of hydrogen cyanide from the hot oxalate treating bath.Another prior art process discloses the use of a combination of asilico-fluoride compound and an organic nitro compound as activators oraccelerators in an oxalate treating bath.

It is therefore an object of this invention to provide an oxalatetreating composition and method for use in the coating of stainlesssurfaces that is greatly simplified over the above prior art processes.

It is another object of this invention to provide an oxalate treatingcomposition and method which effectively coats stainless surfaceswithout the necessity of pro-treatment modification of the surfaces tobe treated.

It is another object of this invention to provide an oxalate treatingcomposition and method which does not require the use of expensive ordangerous compounds.

Other objects and advantages of the composition and method of thisinvention will readily become apparent from a reading of the followingdescription and claims.

It has surprisingly been discovered that clean ferrousbased stainlessmetal surfaces may be effectively coated with an adherent oxalatecoating by contacting said surfaces with a single aqueous solutionconsisting essentially of oxalic acid, ferric ions, bisulfate ions,silicofluoride ions and thiosulfate ions. While the exact mechanism ofthe present invention is not known, it is believed that the use of thesilicofluon'de ion together with the ferric ion results in an effectivebalance between the pickling, or coating-dissolving, action of thebisulfate-silicofluoride complex, and the inhibition of this picklingaction by a common ion effect due to the presence of the ferric ion in alarge proportion. The resulting balance gives an accelerated coatingaction without the metal surface passivation effect usually associatedtherewith, particularly in the presence of the stronger oxidizingagents. Utilization of the composition and method of this inventionresults in the rapid formation on stainless metal surfaces of uniform,densely crystalline adherent coatings having coating weights in therange of from about 700-3000 mgm. per square foot.

Examples of composition suitable for use in accordance with thisinvention preferably have an analysis falling Patented Apr. 21, 1964within the following ranges (parts being expressed as percent by weightof a dry composition) Percent Oxalic acid 50-89 Ferric sulfate 6-18Sodium bisulfate 2-14 Sodium silicofiuoride 2-14 Sodium thiosulfate1.0-4.0

Example 1 A processing tank is cleaned, filled with a quantity of waterand heated to from -130 F. For each one hundred gallons of Water in thetank there is added 28.6 pounds of oxalic acid, 5 pounds of ferricsulfate, 3.2 pounds of sodium bisulfate, and 3.2 pounds of sodiumsilicofluoride. The contents of the tank are stirred to effect completesolution, and the temperature of the bath then raised to F. Just priorto the processing of the stainless articles to be treated, one pound ofsodium thiosulfate for each 100 gallons of solution is added to the thebath. The stainless articles are then immersed in the bath for about 10to 15 minutes while ensuring complete contact of the treating solutionwith all parts of the work. When the articles have received the desiredcoating density, they are removed from the bath, drained, rinsed inwater, dried, and lubricated.

Ferrous-based stainless articles treated in this manner are ready to beplastically deformed in the conventional manner.

During the treatment in accordance with the process of this inventionthe treating solution should be kept at a titration of 40-45 ml. of 0.1N NaOH to a phenolphthalein end point per 5 ml. of solution, and at aniodine equivalent of 1.0-2.0 ml. of 0.1 N iodine solution per 25 ml. ofsolution. The acidity is adjusted by the addition of proper quantitiesof the oxalic acid, ferric sulfate, sodium bisulfate and sodiumsilicofluoride mixture. The iodine equivalent is adjusted by addition ofsodium thiosulfate.

The following examples are illustrative of further specific drycompositions which are eifective in accordance with this invention,parts being expressed as percent by weight:

Example 2 Percent Oxalic acid 75.8 Ferric sulfate 12.2 Sodium bisulfate7.8 Sodium silicofluoride 3.8 Sodium thiosulfate 2.4

Example 3 Oxalic acid 50 Ferric sulfate 18 Sodium bisulfate 14 Sodiumsilicofluoride 14 Sodium thiosulfate 4 Example 4 Oxalic acid 60 Ferricsulfate 15 Sodium bisulfate 11 Sodium silicofluoride 11 Sodiumthiosulfate 3 Example Oxalic acid 80 Ferric sulfate Y Sodium bisulfate 4Sodium silicofluoride 4 Sodium thiosulfate 2 Example .6

Oxalic 'acid 89 Ferric sulfate 6 Sodium bisulfate 2 Sodiumsilicofluoride Q 2 Sodium thiosulfate 1 Having thus described myinvention, I claim:

1. A process for treating the surface of stainless steel based metalswhich comprises the step of contacting the clean metal surface with anaqueous bath containing from about 30 to about 50 pounds per each onehundred gallons of water of a dry composition consisting essentially of:

Percent by weight Oxalic acid 50.0 to 89.0 Ferric sulfate 6.0 to 18.0Sodium bisulfate 2.0 to 14.0 Sodium silicofiuoride 2.0 to 14.0 Sodiumthiosulfate 1.0 to 4.0

2. The process of claim 1 wherein said aqueous bath is maintained at atitration of 4045 ml. of 0.1 N NaOH to a phenolphthalein end point per 5ml. of solution, and at an iodine equivalent of 1.0-2.0 ml. of 0.1 Niodine solution per 25 ml. of solution.

3. The process of claim 2 wherein said aqueous bath is maintained at atemperature in the range of from room temperature to about 155 F.

4. The process of claim 2 wherein said aqueous bath is maintained-at atemperature in the range of from about 145 F. to 155 F.

5. The-process of claim 1 wherein said aqueous bath is maintained at atemperature in the range of from room temperature to about 155 F.

Percent by weight Oxalic acid 50.0 to 89.0 Ferric sulfate 6.0 to 18.0Sodium bisulfate 2.0 to 14.0 Sodium silicofluoride 2.0 to 14.0

(2) adding to said solution just prior to said treatment an amount ofsodium thiosulfate in the range of from about one to about four poundsper each one hundred gallons of water in the solution, and

I (3) then immersing a stainless steel based metal article in saidsolution resulting from step (2), Whereby there is formed on the surfaceof said article a uniform, dense, adherent, crystalline oxalate coating.8. A composition for producing a uniform, dense, adherent, crystallineoxalate coating on the surface of stainless steel based metal articleshaving a composition consisting essentially of:

Water to make 100.0.

References Cited in the file of this patent UNITED STATES PATENTS2,550,660 Amundsen et al May 1, 1951 2,809,138 Wagner et al. Oct. 8,1957 2,976,193 Pimbley Mar. 21, 1961

1. A PROCESS FOR TREATING THE SURFACE OF STAINLESS STEEL BASED METALSWHICH COMPRISES THE STEP OF CONTACTING THE CLEAN METAL SURFACE WITH ANAQUEOUS BATH CONTAINING FROM ABOUT 30 TO ABOUT 50 POUNDS PER EACH ONEHUNDRED GALLONS OF WATER OF A DRY COMPOSITION CONSISTING ESSENTIALLY OF: